In endoscopy, light is used in an area, such as within a human body cavity, that is remote from the actual light source. Xenon light sources have been used but xenon light bulbs are expensive, each costing hundreds to thousands of dollars. Also, xenon light bulbs require replacement every 300 to 500 hours, degrade in terms of light quality over time, and output a significant amount of infrared radiation that may cause unwanted heating of the tissue or object being viewed through the endoscope.
As an alternative to xenon bulbs, light emitting diodes (LEDs) may be used for their higher reliability, electrical efficiency, and color tunability through use of multiple LED colors. A problem with LEDs is that their light output is lambertian, or, more generally, diffuse in nature. Consequently, coupling of LED light output into a small diameter fiber optic of an endoscope is very inefficient and only a small percentage of the LED light output reaches the object to be viewed. Also, many LEDs must be used in an array to produce an equivalent amount or intensity of light as an incandescent or xenon bulb. Focusing the light from a multitude of LEDs in an array to a required spot at the end of a fiber optic cable is complicated and difficult. Multiple lenses would be required, one for each LED, with each lens focusing its own LED to the required spot. Also, fiber optics have a fixed numerical aperture, which is a dimensionless number that characterizes the range of angles over which the optical system can accept or emit light. As the angle of the light entering the optical fiber increases, the percentage of that light that is actually transmitted to the other end of the optical fiber decreases. These factors can make LEDs impractical for the high brightness output required for endoscopes which transmit light through optical fibers.
Lasers may be used as a light source for endoscopes as an alternative to xenon and LED emitters. However, lasers pose other challenges. Physical and electrical properties of semiconductor lasers, for example, require them to operate within fairly narrow bands of electrical current to maintain maximum light output and efficiency. With many types of lasers, the current level needed for maximum light output is beyond the level at which the lasers can be operated continuously without causing thermal damage to the lasers. To address this problem, a laser may be operated with pulses of current. For example, a laser that operates optimally at 60 amps (with a forward drop of 3 volts) but cannot dissipate more than 60 watts may be run at 60 amps for one third of the time. In addition to the aforementioned thermal issue, lasers are sensitive to the frequency at which the current is pulsed. If the current pulse frequency is too low, thermal issues and reduced efficiency result. If the current pulse frequency is too high, efficiency is also adversely affected.
Existing systems use a pulse width modulation (PWM) system to control power to the laser. With a PWM system, the “on” time of the current, or the “off” time of the current, is varied in order to control the percentage of time the current is supplied to the laser. The operating frequency is generally flexible and can be adapted to suite the particular laser being used. A problem with these existing PWM systems is that when implemented in an analog fashion (e.g., using a ramp and a comparator), the resulting accuracy and resolution of the output current to the laser is poor and does not lend itself to control by digital means. To avoid this problem, the PWM could be implemented in a fully digital system, but doing so to achieve high resolution requires a fast clock speed that, although possible, can be prohibitively expensive. For example, to achieve 4096 levels of resolution in the output current, with a typical 500 kHz operating frequency for a laser, would require a 2 GHz clock frequency (about 4096 multiplied by 500,000). Thus, such a fully digital approach limits the available devices for implementing the necessary logic and significantly increases cost compared to a lower frequency system.
Prior art systems 200 for controlling laser output have utilized a variable voltage source 202 that is switched off and on (for the pulsing) with a semiconductor 204 external to the voltage source and connected in series with the laser 206, as shown in FIG. 1. Such an approach results in poorly controlled current through the laser because the current from the voltage source is largely dependant on the load provided by the laser. The load provided by the laser varies as the laser heats and cools, as the laser ages, and due to other factors. Due to inherent variations within lasers, the load also varies from laser to laser. The variation in load causes the current to the laser to vary, making it difficult to maintain load current at levels that produces optimal laser light output.
Another laser driver design 300, as shown in FIG. 2 and disclosed in U.S. Pat. No. 5,748,657 entitled “High Efficiency Constant Current Laser Driver, incorporated herein by reference, has been developed to support constant output current to a laser 302. However, such a design 300 cannot support transient or pulsing output to the laser. Current to the laser is constant, as shown in FIG. 2a. A common buck topology is formed by switches 304 and 306, an inductor 308, and an output capacitance 310. Such a system 300 allows for current mode control. The large output capacitance 310 is used at the buck converter output to conduct inductor AC current ripple, ensuring constant voltage, with very small voltage ripple, at the buck converter output to the laser diode 302. The result is almost constant load current into the laser diode 302. The current into the laser diode 302 is sensed and controlled by adjusting the output voltage of the buck converter. As previously mentioned, transient or pulsing of load current, which is often required for optimal optical output of solid state lasers, is not supported by such a design.
Other prior art systems 400, such as shown in FIG. 3 and often used for driving high power lasers, support pulsing. However, as shown in FIG. 3a, the shape of the load current pulse is not an ideal square wave and is defined by exponentials associated with capacitive discharge, and current level is not constant (has a curved peak) during the pulse. A regulated DC rail (Vdc) is created by the DC/DC converter 402, which may be a buck converter or full bridge. An energy storage capacitor 404 is charged to Vdc via resistors 406, 408, limiting the charge current. A clamp diode 410 provides a charging path and ensures that the laser diode 412 does not develop high voltage above a level that would cause breakdown during reverse biasing. When a switch 414, typically a MOSFET (metal oxide semiconductor field effect transistor), is turned on (closed state), the storage capacitor 40 discharges through the switch, causing a current pulse in the laser diode 412. The resistor 408 limits the amplitude of the current pulse. Another resistor 416 allows current sensing of the pulse amplitude. The combination of the resistance value of the resistor 408, Vdc amplitude, and duration that the switch 414 is turned on allows the amplitude and duration of the current pulse to be adjusted. That is, pulse duration or duty cycle is dependant on circuit values. With this circuit, current is not constant during the pulse, as previously mentioned. The circuit is difficult to control and adjust, and typically cannot be controlled dynamically without risking damage to the laser. Also, as charging and discharging of the energy storage capacitor 404 is done via resistors in a linear fashion, significant power loss in the circuit is unavoidable. Additionally, when the switch 414 is turned on and the system delivers the current pulse to the laser by discharging capacitor 404, during which resistor 406 simply dissipates power from Vdc without any useful function as it is effectively shorted across Vdc by switch 414.
Accordingly, there is a need for a light source for endoscopic use and other uses that is cost efficient, energy efficient, reliable, and provides color tunability. There is also a need for a driver circuit that allows for cost efficient, accurate, and precise load current for a light emitter.